The present invention relates to an endoscopic optical fiber bundle comprising a bundle of optical fibers passed through the insertion portion of an endoscope.
If an endoscopic optical fiber bundle is simply passed as such through the insertion portion of an endoscope (i.e., without any protective sheath), the fibers will get stuck in various areas and break. To deal with this problem, the bundle is sheathed in a flexible tube except in a socket at the end portion.
However, typically in the curved portion of an endoscope that is bent in various directions by remote control, the optical fiber bundle is frequently bent in unspecified directions with small radii of curvature; the resulting strong cyclic flexural stress causes the sheath tube to tear at the boundary with the socket and the sheathed optical fibers may often bend to failure.
An idea to cope with this difficulty is to reinforce the flexible sheath tube over the optical fiber bundle by surrounding it with a helical tube of metal strip at least in the area within the curved portion of an endoscope.
FIG. 6 shows an end and the nearby portion of an endoscopic optical fiber bundle that adopts the above-stated idea. End portions of many optical fibers 91 in bundle are secured into a tubular socket 92.
The optical fibers 91 in bundle are sheathed in a flexible tube 93 an end of which is tightly bound onto the socket 92 which, in turn, is surrounded with a helical tube 94. An end of the helical tube 94 is inserted into a tube 95 tightly bound to the socket 92 and the inserted helical tube 94 is securely joined to the tube 95.
The problem with the endoscopic optical fiber bundle shown in FIG. 6 is that as it is bent repeatedly during the use of the endoscope, the fixed tube 95 is rubbed intensely by the tip of the helical tube 94 and may sometimes tear (in area A).
Then, the intense cyclic flexural stress is concentrated at the boundary (area B) between the sheath tube 93 and the socket 92 until the former tears in that area, causing failure of the optical fibers 91.